The ruins of Machu Picchu are covered in jungle growth in this 1911 photograph taken when Yale archaeologist Hiram Bingham first came to the site a century ago this week.

Bingham was surprised to find that the ancient Inca sites he visited in Peru, including Machu Picchu, weren't as hidden or deserted as he imagined they would be. (Related: "What Was Machu Picchu For? Top Five Theories Explained.")

"When he climbed the mountain on July 24, 1911, he was very surprised to find an Indian family at the top of the ridge," said Christopher Heaney, a Harrington Doctoral Fellow at the University of Texas at Austin.

In fact, three families were living on the mountain ridge on which Machu Picchu was built. A young boy from one of those families guided Bingham up the rest of the mountain, where he got his first complete glimpse of the 15th-century city that he would later make world famous.

Most of Machu Picchu was covered in jungle vines and trees in 1911, but there were a few sites that the Indian farmers had cleared away to grow crops.

"It was very much a living site, not something lost and dead," said Heaney, who is the author of the book Cradle of Gold: The Story of Hiram Bingham, a Real-Life Indiana Jones, and the Search for Machu Picchu.

Today the buildings at Machu Picchu are free of the vines and jungle growth that covered them when Bingham first arrived in 1911. The homes of the indigenous Indian families that Bingham encountered during his first visit are also gone.

Those families didn't live in Machu Picchu's stone buildings but rather in wooden huts that they had built on other parts of the mountain ridge.

"Bingham's arrival really changed their lives, because the landowner now knew there were people living there," Heaney said. The landowner "might have started collecting rent, we don't know. But we do know that there were three families living there in 1911, and by 1915 only one family was left."

That family was eventually hired by the Peruvian government to be caretakers of the site.

Bingham returned to Machu Picchu in 1912 with a team to begin excavation of the site, and then briefly visited again in 1915 (pictured).

During his 1915 trip, Bingham wrote in his journal that he was shocked at how fast the jungle had grown to recover the excavated buildings.

"... nearly wept to see how it had gone back to jungle and brush," he wrote. "Only one group of buildings clear—and that occupied by six pigs! Our lovely camp site now occupied by a hut and dirt, squalor, and filth. Very little of the ruins visible. Alas, Alack-a-day!"

Machu Picchu's ruins are seen neatly excavated in 1997. The unique beauty and historical importance of the site led UNESCO to declare it a World Heritage site in 1983.

During his 1912 trip to Machu Picchu, Bingham and his team excavated some of the buildings and temples in search of artifacts—metalwork in particular.

However, they were mostly unsuccessful. The scientists began finding objects of interest only after members of the farming families living at the site guided them to burial chambers hidden on Machu Picchu's mountain slope, Heaney noted.

The skeletons found in those chambers led Bingham to speculate that Machu Picchu might have been a temple devoted to the Virgins of the Sun, a holy order of chosen women dedicated to the Inca sun god, Inti.

Bingham's theory was based on the incorrect interpretation that the skeletons were mostly female. It's now known that the remains were about half male and half female.

A local man sits on stairs in the entrance to Machu Picchu in this undated photograph taken by Hiram Bingham.

"I would kill to see [Machu Picchu] the way that Bingham first saw it," Heaney said. "It looks so incredible and romantic from the pictures that he took." (See more of Bingham's Mach Picchu pictures from 1913.)

Why the Inca built Machu Picchu is still a mystery. According to one popular theory, it was the royal retreat of the 15th-century Inca Emperor Pachacuti.

"The members of Pachacuti's panaca [royal court] may have lived there during the year for a few days, weeks, or months," Guillermo ****, a Lima-based archaeologist, previously told National Geographic News. (**** has also received funding from the National Geographic Society's Committee for Research and Exploration.)

The same gateway stairs are cleared of jungle growth and debris in a 2007 photograph.

Today, most of the archaeological work that happens at Machu Picchu is focused on the site's conservation, **** said.

"They keep the place in very good shape, but they do not really excavate the pre-Columbian levels of occupation," he added. "To do that, you really have to close down sections of the city for long periods of time."